A boiler is short cycling when the burner goes on and off in short periods of time. Short cycling leads to a lot of wear and tear on the boiler and also leads to higher energy usage. Short cycling is not good!
Short cycling results in the heating load being a lot smaller than the boiler’s heat output. An example would be the burner turning off for 4 minutes and turning back on in 4 minutes in repeating cycles. Many years ago a boiler short cycling was infrequent since heating systems tended to be high thermal mass with only 1 heating zone. Having a zoned heating system leads to higher comfort and should also lead to lower energy usage. However, as previously stated short cycling leads to higher energy usage. Generally speaking we want a boiler to fire at least 15 minutes before shutting the burner off on high limit.
We can end short cycling by changing the design of our heating system. This is a very expensive solution that is not necessary. Adding thermal mass to the heating system will decrease the burner short cycling and this can be done by adding water volume to the system. A relatively easy way to add thermal mass is adding a “Buffer Tank”. A “Buffer Tank” is simply a tank of water.
For buffer tanks, one size does not fit all applications. A buffer tank needs to be sized according to your system requirements. Most manufacturers of buffer tanks also give sizing information. The term “Buffer Tank” is the proper name for this in the hydronic heating industry.
A good heating contractor is trained and competent in adding a buffer tank to a heating system.
Keith Muhlmeister, August 30, 2018